Roll-up doors are used in a variety of industrial applications, typically for the purpose of separating areas within a building, or closing off building entries from the outside. A typical roll-up door comprises a curtain (usually made of fabric) which is wound about a roller journaled for rotation above the doorway with which the roll-up door is associated. To close the door, the roller is rotated such that the curtain pays off of the roller to enclose the doorway. Conversely, the door is opened by reversing the direction of the roller and rolling the fabric curtain onto the roller. Such roller doors are typically either powered open and closed, or are powered open and allowed to fall closed by gravity.
When the roll-up door is placed over an exterior doorway of a building, provision must be made to prevent the fabric curtain from billowing due to wind being applied from the outside. Similarly, when the roll-up door is in place between different sections of a warehouse, there may be pressure differentials between these two sections, which may also cause billowing of the roll-up door if the door does not have provision to prevent this from happening. Such billowing may be problematic as it impedes door function and allows leakage around the door. One technique for preventing such billowing is providing a rigid or semi-rigid bottom bar across the leading edge of the door. Further, other straps extending across the width of the door may be disposed at other positions vertically along the door.
A further technique for preventing billowing of a roll-up door is to exert tensioning forces on the door to hold it taut and provide wind retention. Many of the existing systems for applying this type of tension to a roll-up door make use of a drive belt or other cable or rope having one end attached to the leading edge of the door (such as to a bottom bar), and a second end attached to a drum on the roller shaft. Drive belts are used primarily to pull on the leading edge of the door curtain to draw it closed. According to some existing designs, however, this drive belting is also used for applying tension to the curtain. The drive belting or other cable or rope usually passes over pulleys typically disposed in the side frames on either side of the door. Such tensioning assemblies also include an external moving or spring-loaded member which acts on the belting or cable to exert the tensioning force on the door. For example, a weight may be suspended from the cable or rope between its ends as in U.S. Pat. No. 5,222,541 to Hornberger. In that patent, the weight suspended from the rope also performs a counterbalancing function. In other devices, such as shown in U.S. Pat. No. 4,690,194 to Seuster, the external member tensioning the rope or cable is a leg spring which is disposed between a member fixed for rotation with respect to the roller and a drum upon which the belt is wound which rotates freely relative to the axis of the roller. In yet other examples, such as in U.S. Pat. No. 4,997,022 to Klein, a pulley over which the cable or rope passes is spring-loaded so as to maintain a tension on the strap as the door moves between its unblocking and blocking positions, thus maintaining tension on the curtain to which the strap is attached. Alternatively, other devices are employed wherein such belting is not used for door tensioning during travel, but the door is latched at the bottom in its blocking position and the roller is reversed to exert tension on the door in the closed position.
The exemplary prior art devices just described all require some external member (weights, spring-loaded tensioner, or latches) for the purpose of exerting tension either on the belting, cable or rope or directly on the door. However, such external members may be subject to wear or failure and may thus present maintenance problems or safety hazards. Further, these external members add cost and complexity to the door and may reduce reliability.